   (rand-nth physical-quantities))
 
 (defn rand-unit [physical-quantity]
-  (let [quantity-units-map {:length [:ft :m]
+  (let [quantity-units-map {:density [:lb-per-cu-ft :kg-per-m3]
+                            :length [:ft :m]
                             :mass [:lb :kg]
                             :pressure [:psi :kPa :MPa]
-                            :volume [:bbl :m3]
+                            :proppant-concentration [:lb-per-gal :lb/gal :kg-per-m3 :kg/m3]
                             :slurry-rate [:bbl-per-min :bpm :m3-per-min :m3/min]
-                            :proppant-concentration [:lb-per-gal :lb/gal :kg-per-m3 :kg/m3]}]
+                            :volume [:bbl :m3]}]
+
+
     (fn [] (rand-nth (get quantity-units-map physical-quantity)))))
 
+(def rand-density-unit (rand-unit :density))
 (def rand-length-unit (rand-unit :length))
 (def rand-mass-unit (rand-unit :mass))
 (def rand-pressure-unit (rand-unit :pressure))
+(def rand-proppant-concentration-unit (rand-unit :proppant-concentration))
+(def rand-slurry-rate-unit (rand-unit :slurry-rate))
 (def rand-volume-unit (rand-unit :volume))
-(def rand-slurry-rate-unit (rand-unit :slurry-rate))
-(def rand-proppant-concentration-unit (rand-unit :proppant-concentration))
+
 (defn convert-units-f [from to]
   (let [factors {[:m :ft] 3.28084
                  [:kg :lb] 2.20462262185
                  [:m3 :bbl] 6.28981077
                  [:psi :kPa] 6.894757293168361
                  [:M :k] 1000
-                 [:lb-per-gal :kg-per-m3] 119.826}
+                 [:lb-per-gal :kg-per-m3] 119.826
+                 [:lb-per-cu-ft :kg-per-m3] 16.0185}
         converter {[:ft :m] #(/ % (factors [:m :ft]))
                    [:m :ft] #(* % (factors [:m :ft]))
                    [:lb :kg] #(/ % (factors [:kg :lb]))

     :psi (tuc/draw-normal 7569.89 663.65)
     :kPa (tuc/draw-normal 64.1635 7.0987)
     :MPa (/ (typical-median-pressure :kPa) 1000))))

                   [:kg-per-m3 :lb-per-gal] #(/ % (factors [:lb-per-gal :kg-per-m3]))
                   [:m3-per-min :bbl-per-min] #(* % (factors [:m3 :bbl]))
                   [:bbl-per-min :m3-per-min] #(/ % (factors [:lb-per-gal :kg-per-m3]))
                   [:lb-per-cu-ft :kg-per-m3] #(* % (factors [:lb-per-cu-ft :kg-per-m3]))
                   [:kg-per-m3 :lb-per-cu-ft] #(/ % (factors [:lb-per-cu-ft :kg-per-m3]))
                   [:C :F] #(+ (* % 1.8) 32)}]
    (converter [from to])))

